1. Field of the Invention
The invention relates to a method for the secondary error correction of a multi-port vector network analyzer.
2. Discussion of the Background
For the accurate measurement of complex-value scattering parameters with a vectorial network analyzer (VNA), it is initially necessary to implement a system calibration in the measurement planes of the network analyzer. The measurement planes are generally the ends of the test cables. A plurality of different calibration methods are known for this purpose. TOSM, TRL, LRL, LRM, which differ from one another with regard to the necessary calibration standards and the subsequent evaluation of the individual calibration measurements, can be named as examples. Dependent upon the calibration method and the number of test ports, a measurement of different calibration standards (open-circuit, short-circuit, broadband load, sliding load, (air-) lines, direct through-connection of the test ports) is carried out, from which a determination of the system error parameters is carried out. Using the system error parameters, a numerical error correction is performed in the subsequent measurement of the device under test. This is known, for example, from DE 39 12 795 A1.
After the completion of the error correction, the magnitude of the residual error parameters, which can also be denoted as effective system parameters, is primarily determined, in addition to the methodology of the calibration method and the care taken with the calibration procedure, through the accuracy of the description of the calibration standards, which the manufacturer of a calibration kit supplies to the user. With the introduction of electronic calibration kits, but also with the miniaturization of mechanical calibration standards associated with increasing measurement frequencies, the direct relation to the mechanical properties of the calibration standard has been lost, so that the user is fully dependent on the manufacturer's information. Thus, in addition to the system calibration by means of the named methods, a verification method is therefore often additionally applied, or a secondary calibration is implemented.
In order to estimate the measurement errors based on a potentially erroneous calibration in directive EA-10/12, it is proposed that the absolute value of the effective directivity and of the effective source can be determined by connecting a precision coaxial airline terminated by a mismatch and a short-circuit, respectively, in a defined manner at its output to the test port to be measured by the system-calibrated network analyzer. In order to determine the effective directivity and the effective source match, the magnitude of the reflection coefficient at the input of this air line is measured within the test frequency range. From the oscillations of the magnitude of the reflection coefficient observed in this test configuration, the oscillation amplitude (also referred to as the ripple amplitude) provides a measure for the absolute value of the effective directivity and the effective source match, respectively. However, this method provides only a relatively rough estimate of the effective system parameters, from which a re-correction cannot be performed.
An improved method compared to the above-described for determining the effective directivity and effective source match of the test port of a calibrated network analyzer which is improved is known from WO 03/076956 A2. This method is also based on the measurement of a precision air line short-circuited at the output-end and provides very accurate and moreover complex values for the effective directivity and effective source match, for every frequency point within the test frequency range. The values of the complex reflection coefficient measured within a fine frequency grid are numerically frequency-stuffed, extrapolated, inverse Fourier transformed, and the effective system parameters are extracted by low-pass filtering of the reflectometry signal obtained accordingly in the time-domain. With the determination of the complex-valued, effective directivity and source match, a re-correction can then be applied, and the measurement accuracy can therefore be increased. Furthermore, it is possible to determine the complex-valued effective reflection tracking and accordingly the third system parameter required for the complete description of one-port measurements.
The disadvantage with the known method is that a secondary error correction can only be applied to one-port network analyzers.